The present invention is directed to apparatus for detecting negative ions and more particularly to apparatus where the negative ions are produced by a quadrupole mass spectrometer.
In mass spectrometers and other devices which generate ions, both positive and negative ions are produced. With the new high pressure ionization techniques positive and negative ions are produced in abundance. The chemical composition of the compound under investigation ultimately determines the relative intensity of positive and negative ions. Thus it is analytically useful to detect both polarities of ions.
To detect positive ions a standard continuous dynode electron multiplier (CDEM) is shown in FIG. 1A which is available under the trademark GALILEO as model 4770. The cathode has a voltage of -1 kv to -3 kv impressed upon it. This high voltage accelerates the positive ions into its first stage. The anode end is grounded and the detection signal is obtained at ground potential.
FIG. 1B illustrates a typical negative ion detector of the same configuration as FIG. 1A except that the cathode is operated at approximately +2 kv voltage to attract the negative ions. The output signal at the anode is floated at a relatively high positive voltage from +3 kv to +5 kv.
While the positive ion detector configuration of FIG. 1A is satisfactory the negative ion detector as illustrated in FIG. 1B has several disadvantages:
1. The anode portion where the signal is detected is at a high potential relative to ground requiring an off grounded preamplifier and complex preamplifier circuitry.
2. Since the negative ion detector as shown in FIG. 1B is necessarily a floating system it will be sensitive to stray electrons in the system. Also background noise will be high.
3. Since the output signal lead is at a relatively high positive potential microphonic noise will be severe.
On the other hand, since the positive ion detector of FIG. 1A has its signal output at ground potential it does not suffer these disadvantages and therefore is satisfactory.